The present invention relates to high efficiency collection optics and more particularly to a projection system with a combination of a light source and a reflector having a low maximum deviation angle, with respect to collimated light rays, to ensure collection by the rest of the optics in the system.
The possibility of using liquid crystal displays in projection television is well accepted, and several system have been proposed. In an article on pages 375-377 of the 1986 issue of Society of Information Display Digest, Seiko Epson Corporation discloses a projection system including an illumination subsystem, a modulating device in the path of light emitted from the illumination subsystem, and a projection lens for projecting the image of the modulating device. More specifically, an illumination subsystem in the form of a halogen lamp and a spherical reflector projects light through the condenser lens to a pair of dichroic mirrors which split the light into its red, blue, and green components. Each beam component impinges a respective modulating device in the form of a liquid crystal display (LCD); a dichroic prism combines the three monochromatic images into a single color image which the projection lens projects onto a screen. The article states that the system offers the advantages of compactness, low cost, and brightness. Despite the latter claim, though, the overall light collection efficiency of the system is still less than 1%. That is, for a tungsten halide lamp producing 8800 lumens, less than 60 lumens reach the projection screen. This low efficiency is largely due to the fact that only a small percentage of the light rays is collected and directed toward the modulating device and the entrance pupil of the projection lens.
A further discussion of conventional illumination systems will be helpful. It is well known that a parabolic reflector with a point light source at the focal point of the parabola can provide collimated light beams, and thus offers a potentially high collection efficiency. However, lamps have finite source sizes which result in large deviation angles at the output aperture of a parabolic reflector. Even if a very small lamp is used, light displacements from the focal point can result in additional deviation. Further, when the light valve and thus the reflector are small, it may be impossible to center the lamp at the focal point due to the finite envelope size. The most efficient refractive lens condensing systems are not as efficient (typically less than 43% efficiency) and require expensive multi-element lenses to limit deviation angles. In these refractive lens condensing systems, the envelope size of the lamp is not as important.
Further, when either a parabolic reflector or a refractive lens condensing system used with a rectangular light valve such as an LCD for TV pictures, "fill factor" further diminishes efficiency. For example, for an LCD having a 4 to 3 aspect ratio, only 61% of a circumscribing circle representing the light beam is filled by the LCD. For a 5.33 to 3 aspect ratio as proposed for high definition television, the fill factor is only 54%.
From the foregoing it is apparent that it would be desirable to have a more highly efficient illumination system, that is, a system with far greater lumen output at the output of the reflector as a percentage of lumen output of the lamp. In order to improve the eficiency of the system as a whole, which is the luminous flux at the projection screen as a percentage of lumen output of the lamp, it will also be necessary to keep the maximum deviation angle of light emitted by the illumination system to a minimum. For a large aperture F/2.0 projection lens, this maximum deviation angle would be 15.degree.. Finally, since rectangular light valves in the form of LCDs will be used and it would be desirable to have a 100% fill factor, the illumination system should have a rectangular output aperture corresponding to the shape of the LCD.